In general, vitrified grindstones are used in working operations involving high speed grinding, under various grinding conditions. In cases where grinding is carried out at a relatively high temperature, a water-soluble oil having a good cooling performance is used.
On the other hand, in superfinishing or honing in which a grindstone and a product to be processed are brought into surface contact at a low speed, a water-insoluble oil having a high lubricity and permeability or washability is used, in order to prevent clogging of the grindstone and to facilitate the discharge of cutting chips produced.
A water-soluble oil is used not only for preventing the occurrence of a fire, but also for various reasons regarding productivity and economic efficiency, for example, for intensively collecting the cutting chips produced in chip-generating processes; reducing the area required for installing machines; improving environment, and enabling unmanned operation.
However, in cases where superfinishing is carried out using a water-soluble oil and a vitrified grindstone including fine alumina abrasive grains, it has been empirically recognized that the grindstone is prone to intense clogging shortly after the start of the superfinishing.
In order to appropriately perform superfinishing with a vitrified superfinishing grindstone under the use of a water-soluble oil, it is considered to be a valid countermeasure to use a vitrified grindstone including composite abrasive grains of hard alumina abrasive grains and soft abrasive grains.
Characteristics required for a vitrified bond which holds abrasive grains, such as composite abrasive grains, are: a capability to form a porous grindstone structure, so that the resulting grindstone can exhibit its intrinsic high performance regardless of the type of cooling oil used; and a capability to allow the resulting grindstone to perform grinding with a low abrasion loss, and to provide a finished surface with a sufficiently low surface roughness.
In general, a superfinishing grindstone is produced as follows: abrasive grains, a vitrified bond, and a pore-forming material or a molding auxiliary such as resin balls are mixed, so that the vitrified bond holding the abrasive grains will have a porous structure; the resultant is molded by slip casting or press molding to form a raw grindstone, followed by drying; and then, in a subsequent calcination step, the resultant is heated to a temperature at which the vitrified bond melts and maintained at that temperature for several hours, followed by a finishing step, thereby obtaining the superfinishing grindstone as a product (see the below-identified Patent Document 1).
In the vitrified grindstone produced by the above described production process in which the bond goes through a molten state, pores 4 are formed (see FIG. 2) due to the disappearance of the pore-forming material, such as resin, when glassy bond particles are calcined under normal pressure at a temperature equal to or higher than its melting point. At the same time, the glassy bond particles melt to form bond belts (bond bridges) 5 between the abrasive grains. Thereafter, the bond belts 5 change from liquid to solid in a subsequent cooling process, and the resultant shrinks to reduce the number of pores therein, to finally give a grindstone having a high degree of coupling and a high strength.
Further, a technique is known in which a part of a vitrified binder in an amount of about from 10 to 30% by volume is allowed to melt by a complete melt-solidifying agent, and thereby allowing the binder to hold the abrasive grains with an appropriate hardness, and at the same time, a part of the vitrified binder is sintered so that the sintered portion of the binder moderately degrades during the cutting, to allow for expanding flow paths through which cutting chips are discharged, and for stably securing the flow paths (see the below-identified Patent Document 2).
Still further, a vitrified grindstone is known, which is obtained by subjecting a molded grindstone including a borosilicate glass powder as a vitrified binder, abrasive grains, and carbonaceous spherical particles, to pressure sintering by an electrical heating method at a temperature of a 700° C. or lower, and in which the abrasive grains are held by the resulting sintered product having a porosity of 5% by volume or less (see the below-identified Patent Document 3).
In addition, a low-melting-point inorganic glass is known, which contains from 40 to 48% by mole of SiO2, from 4.5 to 5.5% by mole of Al2O3, from 25 to 32% by mole of B2O3, and from 20 to 27% by mole of R2O+RO, so that the inorganic glass as a vitrified binder melts at a temperature of from 650 to 750° C. during the calcination step in the production of a vitrified superfinishing grindstone including composite abrasive grains (see the below-identified Patent Document 4).